The present invention relates to a variable displacement pump, wherein a cam ring slidable in the pump housing and having variable eccentricity encompasses a rotary piston comprising vanes guided in radial slots. The vanes slide with their head ends at the cam ring, while a discoid annular control disc is interposed between the pump housing containing the inlet and the outlet and the rotary piston accommodated in the pump housing. The control disc includes control slots which correspond with compartments formed between the cam ring, the vanes and the rotary piston and the under-vane chambers, and with the inlet and outlet.
A variable rotary displacement pump is known (German printed and published patent application No. 20 12 536), in which the cam ring is held in the pump housing slidable alongside a supporting surface by means of an adjusting spindle for the purpose of controlling the delivery. It is a disadvantage of this pump that thereis no possibility for precise adjustment of zero delivery, to the effect of precluding any creep rotation of the hydromotor connected to the pump.
It is likewise known to insert short-circuit valves into the pressure lines for the hydromotor by which complete standstill of the hydromotor can be accomplished. However, like devices are comparatively costly and increase the space required for the pump aggregate.